II Edrioaster Buchianus. 



VII. 195 



On the Upper Surface (PL VIII, Fig. 1), five Kadial Grooves 

 proceed fairly straight to a distance of 13 mm. from the actinal pole. 

 They then rather suddenly bend in a dextral or solar direction, and, 

 at about 34 mm. from the 'actinal pole, pass on to the under surface 

 of the theca (PI. VIII, Figs. 2 and 3) ; here they continue along the 

 edge until they have attained a distance of about 46 mm. from 

 the actinal pole, when they terminate. The width of a groove in 

 the proximal half of its course is 5-5 to 6 mm. ; where it bends 

 over to the underside, the width is about 5 mm., and gradually 

 tapers to 2*5 or 2 mm,, after which it is rapidly rounded off. 



Of the five grooves, the anterior is the most perfectly preserved. 

 In its proximal half it forms a rather deep V with slightly concave 

 sides, as shown in section, Fig. 1. On the sides are ridges, passing 

 outwards from the median line at an angle that varies with the 

 curvature of the groove, but is never far from a right angle 

 (PI. X, Fig. 6). These ridges represent the sutures between the 

 flooring-plates. Each ridge passes up to a prominence of somewhat 

 ovoid outline, broader at the end next the groove ; the side of the 

 prominence towards the actinal pole has a gentler slope than 

 the side away from it, so that the prominence as a whole seems 

 to pass outwards in an abactinal direction. The depression between 

 adjacent prominences is continued as an exceedingly slight concavity 

 down the side of the groove to the middle line ; slight though it be, 

 it is observable in some parts where the sutural ridges on either side 

 cannot be detected. The prominences, and consequently the sutural 

 lines, on the two sides of the groove do not actually alternate ; 

 neither do they quite correspond. Therefore the ridge that may, 

 in places, be seen at the bottom of the groove, representing the 

 suture between the flooring-plates of the two sides, bends alternately 

 to either side; in the tract figured (PI. X, Fig. 6), a long stretch 

 towards the right (of the figure) is followed by a short stretch 

 towards the left. 



Sections across the internal cast of the anterior groove, both as seen from the oral 

 side ; x 4 diam. v.g. impression left by flooring-plates of subvective groove. 

 p. matrix rising up into the pore between adjoining flooring- plates. 

 Fig. 1, from proximal region of groove. 



Fig. 2, from more distal region, just on the periphery or ambitus of the fossil. 



The distal region of any groove deviates from the structure just 

 described only in the proportionately less depth of the groove, in 

 the more pronounced alternation of the plates, and in the relative 

 elongation of the prominences, which here merge more gradually into 

 the sutural ridges. (See section of cast, Fig. 2). 



A wax squeeze taken from the impression of the distal region of 

 a groove (e.g. the left posterior, PI. X, Fig. 7) restores the appearance 



